The long-record-length features of modern long-record-length oscilloscopes are generally found to be very difficult and cumbersome to control. That is, when one has collected 32 Mbytes per channel of data in such a deep memory oscilloscope, how does one then use and interpret that data? For example, assume that the user wanted to scroll through the entire record looking for a particular event that caused a problem in the system under test. For such a visual scan, a scrolling-rate of about 500 points per second is quite reasonable. That is, a particular point on waveform would move across the screen from right to left in about 1.0 second. Unfortunately, at this rate it would take approximately 17.5 hours for the user to view the entire data record.
The fact that many oscilloscopes include a printer might lead one to think that the solution to this problem would be to merely print out the entire record. For such a print out, a resolution of 300 points per inch is quite reasonable. Unfortunately, if the user were to print out such a long record on paper at 300 points per inch (approximately 118 points per cm), the printer would use 1.684 miles (2.6944 km) of paper. These two examples highlight the difficulty in dealing with large amounts of data. It simply is not practical for the user to visually inspect all of the collected data for the anomalies that the user must find.
Modern DSOs attempt to solve this problem by waiting for a trigger event to occur, and then acquiring in memory a frame of waveform data surrounding the event. The frame is then processed by waveform math software, measurement software, and display system software. All of this post-processing creates extremely long periods of “dead time”, in which the DSO is incapable of acquiring and storing additional waveform samples. As a result, the anomaly that the user is searching for may occur, and be missed.
More recent DSOs have attempted to reduce the “dead time” by physically positioning Digital Signal Processing (DSP) ICs close to the acquisition memory to convert acquired waveform data to display data more efficiently. This arrangement is sometimes referred to as a “FastAcq” mode of operation. Use of FastAcq circuitry has greatly reduced the “dead time” between triggers, and increased the number of samples per second that are displayed. Unfortunately, the data frames processed by the FastAcq circuitry are not retained, and are therefore unavailable for additional processing. Moreover, cycle-to-cycle measurements (for jitter measurement) are adversely affected by the use of FastAcq circuitry because the time relationship between successive triggers is not maintained.
Another disadvantage of many current DSO architectures is a “bottleneck” that exists because they transfer all of the data from acquisition memory to main memory for processing and display over a relatively slow (i.e., typically 30 Mb/sec.) data bus.
In order to process this transfer-rate issue, Agilent Technologies, Inc. of Palo Alto, Calif., has recently introduced Infiniium MegaZoom deep-memory oscilloscopes employing a custom ASIC that optimizes the sample rate for a given sweep speed and sends only the waveform data needed for a particular front panel setting. The Infiniium MegaZoom oscilloscopes provides a waveform update rate that is approximately twenty-five times greater than conventional deep memory oscilloscopes.
Wavemaster™ oscilloscopes with X-Stream™ technology, manufactured by LeCroy Corporation of Chestnut Ridge, N.Y. provide an alternative solution to the transfer-rate problem. These oscilloscopes employ a silicon-germanium (SiGe) digitizer and a high-speed streaming bus to transfer data from an analog to digital converter (ADC) through an acquisition memory and into a memory cache for extraction of information by software routines.
None of the above-mentioned systems provide the solution to the problem of quickly identifying anomalies and transferring only a frame of memory surrounding the anomaly to the waveform processing and display portions of the oscilloscope.